1803. } Confiderations on the 
‘ procure obfervations ; and I have been 
enabled to make Tables of Mercury which 
accord always with the heavens toa quar- 
ter of a minute. 
It had been remarked that the Moon 
had a movement more rapid than former- 
ly. Citizen De La Place found the caufe 
to be the attraction which the planets ex- 
ercife over the -Earth, and which, chang- 
ing the pofition of its orbit, changes alfo 
the attraction which the Earth exercifes 
over the Moon. 
Saturn had in its movement a retarda- 
tion of more thana week, which I had ob- 
ferved in 1769. Citizen De La Place 
found, in 1786, that the attraction of Ju- 
piter was the caufe of it ; and Citizcn 
Delambre has made tables which are ftill 
accurate to half a minute. 
The Satellites of Jupiter have been one 
of the moft difficult parts of aftronomy ; 
but, at laft, the Theory of Citizen De La 
Place, and the Tables of Citizen Delambre, 
have brought the(e planets to the fame pre- 
cifion as the others. 
The enumeration of the ftars is the 
principal foundation of aftronomy. I had 
paffed the thirty firt years of my life in 
planetary af'ronomy ; I wifhed to devote 
the laft to the ftars. Three thoufand had 
been obferved before me; I have been en- 
abled to trace fifty thoufand, with the help 
of my nephew; an immenfe work, which 
no one durft undertake or hope for. 
The inftruments have acquired a degree 
of perfection in like manner unhoped for. 
Dollond, agreeably to an idea of Euler, 
made achromatic-glaffes, that is to fay,’ 
without colour, which have a much more 
confiderable effect than the antient ones.—= 
Bird andRamfden made inttruments divid- 
ed with a precifion altogether novel ; and 
Lenoir, at Paris, difputes the palm with 
all the Englifh artifs. 
Herfchel made a telefcope of twenty 
feet, which enabled him to difcover, in 
1781, a new planet, the fix Satellites which 
{urround it, two Satellites of Saturn, and 
the rotation of his ring. Caroché made 
ene at Paris of the fame powers; but 
Her(chel has fince made one of forty fret, 
which, perhaps, will produce new difco- 
veries. Mayer invented a method of mul- 
tiplying the refults of an obfervation on 
all the points of a circle, fo as to obtain 
an accuracy ten times greater than was 
known before. This ingenious idea, 
which the Chevalier De Borda has put in 
practice at Paris, has produced another 
revolution in aftronomy. 
Hlarrign made marine-watches. which 
Progrefs of Afironomy. 
do not vary two minutes in two months 
navigation. Leroi and Berthould have 
made fimilar ones in France ; and now it 
is actually poffible to make a tour of the 
terraqueous globe without entertaining the 
leaft doubt as tothe longitude. To know 
what the hour is on board a veftel, there 
is not even need of calcnlation ; the Hora- 
ry-tables that Madame De Lalande has 
publifhed give the hour toa fecond in all 
countries of the globe. Clocks have 
been brought to fuch a degree of perfec- 
tion, as not to vary a fecord per day in 
the courfe of a year; I have even one 
which has been forty years at the fame 
fecond, 
Encouraged by the perfeétion of the in- 
ftruments, altronomers have* acquired {fo 
conftant and familiar a®ufe of them, that 
they can divide a fecond of time into ten 
parts without erring one-tenth, 
When the Conftituent Affembly had 
decreed,-in 1790, that there fhould be 
only one meafure in France, in order to 
terminate, at length, the ttrange compli- 
cation which had crept into all the pro- 
vinces, the Academy judged it neceflary to 
eftablifh fo natural a meature that all civi- 
lized nations might adopt it. For this 
purpofe the magnitude of the earth was 
fixed upon. It became neceffary, there- 
fore, to make a new meafurerment of de- 
grees from England to Spain. Delambre 
and Mechain fpent many years in this valt 
operation, which has fhewn to us the real 
magnitude of a degree of the earth, and 
confequently the real greatnefs of the me- 
tre, or of the univerfal meafure, 36 inches, 
11 lines, and 3 tenths of onr antient toife. 
But this labour has, morecver, procur- 
ed to usa new article of information ; that 
is, the irregularity of tne Eayih, which is 
found to be more curved towards the mid- 
dJz of France than it .is in the totality of 
its circumference. It is curved a ssoth 
partin France, whilft, from our coun:ry 
to America, it is only a 334th. This ir- 
regularity occafions a great inconvenience 
in aftronomy ; but it is a truth which it 
is important fer us to know. 
The general theory of attra&tion has 
made in this half-century an immenfe pro- 
gref> from the refearches of the great geo- 
meters it has produced, fuch as Euler, 
D’ Alembert, Clairaut, Lagrange, and La 
Place. The effects of attraétion have Leen 
multiplied in the eyes of aftronomers ; the 
attraction of mountains on terreftrial bo- 
dies has been afcertained and meafured in 
Peru, in France, in Italy, and in Scotland; 
that of the terreftrial bodies has been alfo 
Q2z meafured 
115 
